Field of the Invention
The invention relates to a circuit configuration for limiting the output voltage of a clocked voltage regulator or controller, wherein the clocked voltage regulator includes a control unit receiving a direct voltage to be regulated and having an output supplying the output voltage; a voltage regulating amplifier being constructed as an integrator and receiving the output voltage and a reference voltage; a multiplier multiplying the output signal of the voltage regulating amplifier by a signal obtained from the direct voltage to be regulated; a current regulating amplifier receiving the output signal of the multiplier and a signal obtained from the direct voltage to be regulated; and a driver stage converting the output signal of the current regulating amplifier into a trigger signal for the control unit.
A circuit configuration for limiting the output voltage of a clocked voltage regulator is known, for instance, from the publication entitled: Siemens Components 31 (1993), No. 2, p. 46 ff. FIG. 2 on page 48 thereof shows a regulator structure with a multiplier between a voltage and a current regulator. As a result, a curved shape of the input voltage is imposed upon a command or set point value for the current. In that way, an optimal curved shape is attained for the mains current being consumed, even in the partial-load range, and that shape is preserved even if the peak value of the input voltage approaches the output voltage. By using that kind of regulator structure, it is easy to achieve radio interference suppression. However, the voltage regulation must be slow if a power factor as close as possible to 1 is to be obtained, and that causes dynamic voltage overswings at the output which can threaten subsequent components in the circuit.
The literature, in an article by C. Nelson and H. Lemme entitled: "Bessere Schaltnetzteile durch Leistungsfaktor-Korrektur" ["Better SPMSs through Power Factor Correction"], in Elektronik 23/1993, pp. 86-89, discloses a circuit configuration of a clocked voltage regulator using the integrated LT1248 component. Besides the aforementioned circuit configuration, it also includes a voltage regulating amplifier, constructed as an integrator, and forms the point of departure for the invention. However, the circuit requires one additional pin. Moreover, a no-load protection is provided, which causes shutoff of the end stage and can thus cause noise to develop in use.
Other prior art shows various provisions. For instance, the known L6560 component made by SGS Thomson achieves overvoltage detection by ascertaining the current in an integration capacitor. However, that kind of provision offers no no-load protection, and the digital switchoff means for the overvoltage circuit can cause noise to develop in the consumer in the event of overvoltage.
An overvoltage protection circuit made by the company known as Linear Technology is also known, and is realized in its LT1249 component. That circuit offers no-load protection with a comparator to which a reference voltage is supplied. That comparator controls the output signal of the multiplier and monitors a current threshold and the switching performance of the multiplier input. In that kind of configuration, noise can still be produced in the consumer when overvoltage occurs.
All of the aforementioned overvoltage protection circuits essentially have only a simple comparator, which digitally shuts off the driver output, in other words the triggering of the control element, if overvoltage occurs. In such regulators with a fixed operating frequency, that kind of digital overvoltage shutoff intervenes completely asynchronously, with respect to both the operating frequency and the mains frequency. As a result, due to superposition of frequencies, distortion of the input current with a high proportion of harmonics occurs, and that also includes components which are non-harmonic (with respect to the mains frequency). That not only misses the target of sinusoidal power consumption, but components, such as ferrite cores, which are capable of acoustic transmission also cause irritating noise.
U.S. Pat. No. 4,837,495 shows a voltage regulator that is said to have a very good power factor. For that purpose, the voltage regulator operates by the current ramp principle, and it uses a compensation circuit for the current ramp. An integrator is also provided as a voltage regulating amplifier.